1. Technical Field
This invention relates to magnetic resonance imaging (MRI) techniques for imaging a first nuclear magnetic resonance (NMR) species while suppressing the imaging of a second, different NMR species using a multi-spin echo MRI data acquisition sequence.
2. Related Art
In the following discussion, some acronyms and/or abbreviations are utilized. Although most, if not all, are familiar to those skilled in the art, a short listing of at least some of these acronyms/abbreviations is set forth below for convenience.                BASING=BAnd Selective INversion with Gradient dephasing        MEGA=MEscher GArwood (the authors)—a technique very similar to BASING        SAR=Specific Absorption Rate (of RF energy)        FSE=Fast Spin Echo—a common MRI pulse sequence        FASE=Fast Asymmetric Spin Echo (similar to FSE)        FOV=Field of View        SNR=Signal-to-Noise Ratio        B0=main static magnetic field        B1=transmitted RF magnetic field        T1=longitudinal NMR recovery time constant        T2=transverse NMR decay time constant        TR=repetition time between successive excitations for MRI imaging data acquisition sequence        ppm=parts per million (unit of measure of frequency offset or deviation)        CHESS=CHEmically Selective Suppression        SPAIR=SPectral (-ly selective) Adiabatic Inversion Recovery        SPIR=SPectral (-ly selective) Inversion Recovery        STIR=Short Tau Inversion Recovery        PASTA=Polarity Alternated Spectral and spaTial Acquisition (excite and refocus pulses have opposite gradient polarities)        DIET=Dual Interval Echo Train (initial echo interval is longer than repeated echo train interval)        WFOP=Water-Fat Opposed Phase        IDEAL=a manufacturer's commercial name for a Dixon-like fat suppression method.        
Lipid suppression in MRI is critical to achieve good image quality in regions where lipid signal obscures anatomy of interest. There exist several techniques designed to leverage the chemical shift difference between water and lipid (4.67 ppm v. ˜1.3 ppm) including spectrally-selective saturation (CHESS) and alternate polarity slice select gradients to only refocus water signal (PASTA). Another family of techniques separates water from lipid using the phase evolution of transverse magnetization due to chemical shift difference (Dixon, IDEAL, WFOP). Somewhat related are spectral-spatial binomial RF pulses to selectively excite water and exclude lipid. Other techniques take advantage of the shorter T1 of lipid (˜250 ms v. ˜1000 ms water) to null lipid signal (STIR, SPIR, SPAIR). Still other techniques use the short T2 and strong J-modulation of lipid to attenuate lipid using irregular echo spacing (DIET).
The values for lipid chemical shift and T1 expressed above are quoted for the dominant methylene (CH2) component of lipid. However other lipid components such as methyl groups (CH3) and CH2 protons shifted by proximity to carbonyls or double bonds are also present. In addition, there are olefinic protons at 5.3 ppm (i.e., relative to the tetramethylsilane (TMS) standard defined as 0.0 ppm), but these are too close to water (e.g., water at 4.7 ppm is only spaced 0.6 ppm from olefinic protons at 5.3 ppm) for any chemical shift-based technique to remove them. Therefore, lipid T1s can range over 100's of milliseconds and chemical shifts can range over 1.5 ppm. For more discussion, see Kuroda et al, “Optimization of Chemical Shift Selective Suppression of Fat”, Magn. Reson. Med. 1997; 40:505-510. In addition, in a clinical MR experiment both B1 and B0 homogeneity can be non-uniform over the FOV. Especially at high field (e.g., 3 T), the range can be as great as +/−40% for B1 and 100's of Hz for B0 depending on the local anatomy.
Each MR imaging lipid suppression technique has advantages and disadvantages that make it suited to a particular application given the experimental B1 and B0 homogeneity and sequence constraints. The general advantages and disadvantages are summarized in the table of FIG. 8.
The BASING/MEGA technique in spectroscopy provides good T1-insensitivity, B1-insensitivity, time efficiency, and chemical shift robustness with minimal sequence restrictions. However, using this technique for imaging in its original spectroscopy manifestation (inverting fat and crushing its signal with opposing gradients while not refocusing water) would create a very long first echo. Furthermore, the RF pulse designs typically used in this technique can, as a function of offset frequency, alter the phase of the spins that are supposed to be left unaffected. Therefore, it is necessary to apply a matched pair of these rf pulses to unwrap the phase variation in order to produce a uniform phase profile over the width of the water resonance.
Some of these prior art techniques are described in references such as the following:    Dixon, “Simple proton spectroscopic imaging,” Radiology, 1984; 153:189-194    Bydder, et al., “The Short T1 Inversion Recovery Sequence—An Approach to MR Imaging of the Abdomen” in Magn. Reson. Imaging, 1985; 3:251-254    Yeung, at al., “Separation of true fat and water images by correcting magnetic field inhomogeneity in situ,” Radiology, 1986; 159:783-786    Meyer, et al., “Simultaneous spatial and spectral selective excitation,” Mag. Res. Med., 1990; 15:287-304    Glover, “Multipoint Dixon technique for water and fat proton and susceptibility imaging,” J. Mag. Res. Imag. 1991, 1:521-530    Glover, et al., “Three-point Dixon technique for true water/fat decomposition with B0 inhomogeneity correction,” Mag. Res. Med., 1991; 18:371-383    Kanazawa, at al., “Contrast naturalization of fast spin echo imaging: A fat reduction technique free from field inhomogeneity” in PROC., SMR, 2nd Annual Meeting, San Francisco, 1994; page 474    Butts, et al., “Dual echo DIET fast spin echo imaging” in PROC., SMR, 3rd Annual Meeting, Nice, 1995; page 651    Miyazaki, et al., “A polarity altered spectral and spatial selective acquisition technique” in PROC., SMR, 3rd Annual Meeting, Nice, 1995; page 657    Kanazawa, et al., “A new fat-suppressed fast spin echo technique using DIET-PASTA” in PROC., ISMRM, 4th Annual Meeting, New York, 1996; page 1547    Mescher, et al., “Water suppression using selective echo dephasing” in PROC., ISMRM, 4th Annual Meeting, New York, 1996; page 384    Mescher, et al., “Solvent suppression using selective echo dephasing,” J. Mag. Res., Series A, 1996: 123:226-229    Kuroda et al, “Optimization of Chemical Shift Selective Suppression of Fat”, Magn. Reson. Med. 1997; 40:505-510.    Block, et al., “Consistent fat suppression with compensated spectral-spatial pulses,” Magn. Reson. Med., 1997; 38:198-206    Star-Lack, et al., “Improved water and lipid suppression for 3D PRESS CSI using RF band selective inversion with gradient dephasing (BASING),” Mag. Res. Med., 1997; 38:311-321    Schick, “Simultaneous highly selective MR water and fat imaging using a simple new type of spectral-spatial excitation,” Mag. Res. Med., 1998; 40:194-202    Ma, et al., “Method for efficient fast spin echo Dixon imaging,” Mag. Res. Med., 2002; 48:1021-1027    Reeder, et al., “Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL): application with fast spin-echo imaging,” Mag. Res. Med., 2005; 54:636-644    Reeder, et al., “Water-fat separation with IDEAL gradient-echo imaging,” J. Mag. Res. Imag., 2007; 25:644-652    Lauenstein, et al., “Evaluation of optimized inversion-recovery fat-suppression techniques for 12-weighted abdominal MR imaging,” J. Mag. Res. Imag., 2008; 27:1448-1454    Bley, et al., “Fat and water magnetic resonance imaging,” J. Mag. Res. Imag., 2010; 31:4-18    Jensen, et al., U.S. Pat. No. 5,142,231.    Miyazaki, of al., U.S. Pat. No. 6,320,377
For example, the above-cited Block, et al. paper describes an example of the use of spectral-spatial binomial RF pulses to selectively excite water and exclude lipid.
In MR spectroscopy, the BASING and MEGA techniques have been used to edit water and/or fat from spectra. These techniques work by including spectrally-selective 180° RF pulses (e.g., see FIG. 2) that invert water and/or lipid resonances and leave metabolites of interest (1.9-3.6 ppm) undisturbed. A pair of alternate polarity Gx or Gy gradient crushers surrounds each spectrally selective RF pulse (e.g., as shown in FIG. 2) to spoil only inverted magnetization. Since these techniques operate on transverse magnetization, they are inherently T1-insensitive.
The design of narrowband RF excitation pulses for multi-slice imaging is also known. See, for example:    Murdoch, et al., “Computer-optimized narrowband pulses for multislice imaging,” J. Meg. Res., Vol. 74, pages 226-263 (1987).
In spite of all this prior work on separating water and lipid MR images, there continues to be a need for a more B1-robust and T1-robust MRI technique for lipid suppression.